There's a CGI demo on their website.

I'd guess that YC advised them to launch now, maybe to get the idea in front of any people (employees, advisers, customers, etc) who could accelerate the concept.

And YC would be betting that their experience counts for something beyond just a random person with a weekly idea:

"We are hardware engineers with over two decades of experience between us, working at Tesla, SpaceX, JPL, Relativity, and some startups."

Personally, and I know that is outside the SV bubble of experience, I would have expected at least one founder with a solid background as a naval architect and another one with an equally solid background in container shipping.

The experience you listed read more like name-dropping PR to sell this whole thing to other investors down the line.

Or maybe some sailing experience. It didn’t sound to me that they are active sailors.

Will there be a keel or any other foils to help create lift to not just get blown sideways?

To me this seems like solar roofs on electric cars. An obviously good idea that doesn’t really have a meaningful benefit.

And it is not even the first time someone tried.

Container ships don't have keels or other foils like real sailboats do. That wouldn't really be possible due to limited hull space and draft. But the hulls are somewhat resistant to leeway on their own.

The various supplemental sail technologies being tried to improve merchant shipping fuel efficiency are mostly only effective going downwind. They have to make serious design compromises compared to a dedicated sailing rig in order to meet cost and space constraints. But with the cost of fossil fuels expected to continue increasing (eventually including perhaps some sort of carbon tax or emissions cap) even a minor increase in efficiency is worth pursuing.

My guess is that some sort of deployable kite sail will eventually win out instead of this wingsail concept. We'll probably start to see that integrated into new ship designs as the cost is pretty low and the equipment occupies a fairly small volume at the bow.

And when those sails are integrated, which is far from certain, I somehow have my doubts the supplier will be some scrappy start up.

They obviously have nothing except the idea. However, you are being wholly unfair to them expecting a prototype. Even with the entire YC funding they won't be able to build more than a scale model, no chance for a working prototype.

This is a very cool, very hard idea to execute - i shudder to think at the kind of testing, reliability and certification you need to ensure a foldable ten-storey building is safe around human crew in gale force winds. It's a moon shot, and sometimes these take you on the Moon.